1. Field of the Invention
This invention relates to an electrochemical process and more particularly to an electrochemical process for the production of sulphuric acid and sodium hydroxide from an aqueous solution of sodium sulphate. In a further aspect the invention relates to an electrochemical cell in which to operate the process of the invention.
2. Description of Related Art
Many industrial processes involve the neutralisation of either sulphuric acid or sodium hydroxide. These processes generally produce as a by-product sodium sulphate which is of little commercial value. An example of this type of process is the production of reconstituted cellulose where for every tonne of cellulose film or fibre produced, slightly more than one tonne of waste sodium sulphate is discarded. Many other processes involve the production of sodium sulphate as a by-product.
Electrochemical processes are known for the splitting of sodium sulphate into its component acid and base, sulphuric acid and sodium hydroxide.
Known electrochemical processes for the production of sulphuric acid and sodium hydroxide from aqueous sodium sulphate solutions are of two types, so-called "electrodialysis" and electrolysis. In the electrolysis type process, the electrode reactions: EQU Cathode: 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- and EQU Anode: 2H.sub.2 O.fwdarw.4H.sup.+ +O.sub.2 +4e.sup.-
are used to split water, and ion-selective membranes are used to keep the ions apart. The process may be carried out in a two compartment cell, a three compartment cell or a cell having more than three compartments.
In the two compartment cell, an anion selective or cation selective membrane may be used to separate the anode compartment of the cell from the cathode compartment of the cell. Where a cation selective membrane is employed, an aqueous sodium sulphate stream is fed into the anode compartment of the cell where it is converted to sulphuric acid and oxygen and sodium ions migrate across the membrane to the cathode compartment where sodium hydroxide and hydrogen are produced. Where an anion selective membrane is employed, the aqueous sodium sulphate solution is fed to the cathode compartment of the cell and the sulphate ions migrate across the membrane to the anode compartment where sulphuric acid and oxygen are produced, a sodium hydroxide solution and hydrogen being produced in the cathode compartment.
In a three compartment cell which has a central compartment, an anode compartment and a cathode compartment, the aqueous sodium sulphate solution is fed into the central compartment. The central compartment is separated from the anode by an anion selective membrane and from the cathode by a cation selective membrane. When current is passed through the cell, sulphuric acid is produced in the anode compartment and sodium hydroxide in the cathode compartment, the sodium sulphate in the central compartment being depleted by an equivalent amount.
In the so-called electrodialysis process the cell comprises a series of alternating bipolar membranes and ion exchange membranes between a terminal anode and a terminal cathode. The bipolar membranes are used to split water into H.sup.+ and OH.sup.-, which are separated to opposing sides of the bipolar membranes under the influence of the electric field. In an electrodialysis process employing, for example, a cation-exchange membrane, aqueous sodium sulphate is charged to the anolyte side of the bipolar membranes and the sodium ions migrate through the cation selective membrane to the catholyte side of an adjacent bipolar membrane where sodium hydroxide is produced, sulphuric acid being produced at the anolyte side of the bipolar membranes.
However these known processes do suffer from disadvantages. Thus the current efficiency of the process, measured in terms of the ratio of Na.sup.+ ions to the total Na.sup.+ ions and H.sup.+ ions transferred across the cation selective membrane, may be disadvantageously small and the concentration of sulphuric acid produced by the process may not be sufficiently great as to make the product commercially acceptable, due to the transference of H.sup.+ across the membrane.
The aforementioned known processes are operated at temperatures up to 60.degree. C. using as anolyte, aqueous sodium sulphate solutions having concentrations of up to 30 wt % sodium sulphate. A 30 wt % sodium sulphate solution is a saturated sodium sulphate solution under neutral conditions at the temperatures employed in the known processes.